Ventilation systems in many buildings are provided with fire dampers at various points in the system. Such fire dampers typically include movable louvers which are either open, allowing free flow of air, or closed, thereby preventing airflow through the associated duct work. A fire damper of this nature is typically operated by an electric actuator. In addition to being able to open and close the damper, the actuator for a fire damper must be configured to permit the associated damper to close from an opened position in the event of loss of electrical power to the actuator. This is typically accomplished by having the actuator "wind up" and tension an associated closing spring while the actuator is opening the damper, with the spring being capable of closing the damper in the event of power loss. Thus, the actuator maintains the damper in an opened position with the associated closing spring "wound up", whereupon a loss of power causes the actuator to release the damper and spring, permitting the spring to close the louvers of the damper. The closing spring may be part of the damper assembly, or may be incorporated in the damper actuator.
As will be appreciated, a number of important considerations must be kept in mind in designing a fire damper actuator mechanism. Since the ventilation system of a building may require many such actuators, an actuator must be economical to manufacture and use if it is to be commercially viable. Thus of concern is the amount of electrical power consumed by the actuator while it continuously holds the damper open. Also of concern is the noise generated by the actuator as it holds the damper in its opened position. While a fire damper actuator may be required to hold the associated damper in an opened position for many months, the actuator must reliably permit the damper to close in the event of power loss after holding it in an opened position for an extended period.
Naturally, versatility of use of a particular actuator requires it to be adaptable for use in association with a high temperature heating duct, a low temperature air conditioning duct, or in a relatively humid air return duct. Durability of an actuator is also important since it may be required to cycle the associated damper from closed to opened positions thousands of times, and must be sufficiently strong to withstand any shock loading attendant to automatic closure of the damper by its closing spring. Finally, in order to be sufficiently versatile to be cost effective in use, an actuator should be capable of operating dampers which have greatly differing input torque requirements, such as being capable of operating a damper requiring 12 inch-pounds of torque as well as a damper requiring 120 inch-pounds of torque.
Presently available fire damper actuators are of two general types. The first comprises a relatively low-torque electric motor which operates through a high reduction gear drive train. The low speed output of the drive train is directly coupled to the damper. This type of device functions to open the associated damper, with power to the electric motor then reduced so that the motor produces sufficient torque to hold the damper in its opened position against its closing spring. Not only must the gearing be configured so that the closing spring can close the damper and also turn the motor backwards, the unit must also withstand the relatively severe shock loading which occurs when the damper "slams" to its closed position under the action of its closing spring, thus quickly stopping the reverse spinning motor rotor.
This first type of actuator suffers from a number of distinct disadvantages. Typically, a relatively large number of gears are required in the gear reduction portion of the actuator, adding to its initial expense. Additionally, the motor of the actuator requires relatively large amounts of power to maintain the damper in its opened position, since relatively inefficient shaded pole alternating-current motors are typically employed. Because of the typically high gear reduction, a relatively small amount of drag on the rotor shaft of the motor can prevent the closing spring from properly closing the damper in the event of power failure. Since the motor is turned backwards at a fairly high speed during closing of the damper, relatively high shock loading occurs at the first-stage reduction gear teeth as the damper moves into its fully closed position as a result of the rotational inertia of the motor rotor. Damage to the gear teeth can result. Since this type of actuator typically employs an alternating current motor, a hum results from the continuous supply of power to the motor coil as it holds the damper open. This hum can be objectionably noisy.
Other disadvantages associated with this first type of actuator include the manner in which the damper is held in its open position. In some arrangements, the motor holds the damper open against a mechanical stop, but in such arrangements, the gear reduction drive train is subjected to more torque than necessary. In some arrangements, the torque output of the motor is balanced against the force of the damper closing spring, but this can result in the position of the damper changing, since the characteristics of the spring or the torque output of the motor may vary somewhat over time.
The second type of fire damper actuator which is presently available also comprises an electric motor which operates through a multi-stage gear reduction unit, but it also includes an electrically activated direct-current clutch which connects the actuator to the fire damper and its closing spring. This type of actuator further includes an electric brake. The brake operates on the rotor shaft of the motor and is normally engaged. This type of actuator operates to open the damper by supplying power to the electric clutch (which engages the clutch) and to the motor (which automatically disengages its brake). The motor then operates through the gear reduction unit to open the damper against the action of its closing spring. When the damper is fully opened, power to the motor is turned off, which re-engages its brake, while power to the electric clutch is maintained. Thus, in essence, the clutch connects the damper to the now-braked motor rotor shaft (via the gear reduction unit) to maintain the damper in an opened position. In the event of power loss, the clutch disengages the damper and its closing spring from the actuator, permitting the damper to close.
This second style of actuator also suffers from a number of distinct disadvantages, principally relating to the required electric clutch. Not only is such a clutch relatively large in size, it must be configured to carry widely differing torques depending upon the torque requirements of the associated damper. Additionally, such an electric clutch is relatively expensive, and draws fairly substantial amounts of power when it is engaged and maintains the fire damper in an opened position against its closing spring.
In view of the distinct disadvantages associated with previously known fire damper actuator devices, it is desirable to provide an improved fire damper actuator which overcomes the disadvantages associated with previous designs. To this end, the fire damper actuator of the present invention has been particularly configured for economical manufacture, reliable and versatile use, low power consumption when operating to maintain an associated damper in an opened position, as well as low noise output when holding the damper open.